The biochemistry of the repression mechanism in histidine biosynthesis in Salmonella typhimurium would be characterized. Crude extracts of bacteria grown under different nutritional states would be examined by sucrose density gradient ultracentrifugation for the competency of the first enzyme of the pathway, adenosine triphosphate phosphoribosyltransferase, to bind to histidyl-tRNAHis. Any change in competency would be characterized as to in vivo requirements for a switchover from a nonbinding to a binding state. Preparative quantities of these two components and of histidyl-tRNAHis synthetase would be isolated and their mutual interactions characterized. Molecular weights and stoichiometry of complexes formed among these three components, and kinetic behavior of the two enzymes while complexed would be determined. Rates of association and dissociation, dissociation constants and ligand and solvent effects on these, and symmetry and conformation behavior of complexes would be characterized. The specificity of the interactions would be studied using purified mutant components and tRNAHis derivatives, and the regions of interaction between the three determined using photocrosslinking. Distances between functionally identifiable sites would be determined by enhanced nuclear relaxation techniques. Binding of these proteins to his-tRNAHis would be determined by nitrocellulose filter techniques or protein fluorescence quenching. Characterization of the biochemical interrelationships of these regulatory macromolecules will help delineate new types of genetic control mechanisms and should be of importance to problems in cancer mechanisms.